Phoebe Waller-Bridge and Kris Marshall are the current front-runners but the BBC has insisted that no cast decision has been made yet.

It was always expected that Capaldi would say his final farewell in the festive episode but the latest trailer for the season finale showed him at the start of his regeneration process sparking speculation it could happen this weekend.

But showrunner Steven Moffatt - who is also walking away from the show this year and handing over the reins to Broadchurch creator Chris Chibnall - has confirmed the Twelfth Doctor will fight his fatal injury to live on.

Moffatt told Doctor Who Magazine: "The doctor is mortally wounded, staggering out of the Tardis into the snow and starting to regenerate.

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But he manages to get inside the Tardis and siphon off his regeneration energy into his severed hand which was kept in a jar inside his space craft.

Capaldi will definitely regenerate in the Christmas Day story, in which he is rumoured to be joined on an adventure by The First Doctor, who will be played by David Bradley.

Bradley, 75, played William Hartnell - the first actor to portray the iconic character - in 2013 drama An Adventure in Space and Time, a dramatisation about the creation of Doctor Who.

Fans have already seen the Time Lord's companion Bill Potts - played by Pearl Mackie - be converted into a Mondasian Cyberman, and the 30-year-old actress admitted it was very "emotional" shooting the plot because so much is about to happen.

Asked if it was "emotional filming the death of [her] character", she said: "I mean, I'm quite an emotional person, so it was pretty emotional.